Regardless of my feelings for Kerry...
The one advantage of someone like him is that his aspirations for the presidency have evaporated. He handily won his senate seat and is locked in for another 6.
So he is in a position where he can be aggressive with little to lose.
The swift boat thing was unique, imo. It wasn't a political challenge, but rather a classless personal affair that could *never* be addressed in a satisfactory way. I think handling actual foreign affairs would be completely different.
The "nothing to lose" clause also comes into play for McCain as well, provided he abandons 2008 maverick and returns to 2000 maverick. He has a good chance of winning his senate seat again, and is done running for president.